Antimicrobial resistance: metallo-beta-lactamase: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Antimicrobial resistance: metallo-beta-lactamase

Sample item

A meropenem-resistant Klebsiella pneumoniae is isolated from an ICU patient. In a combined disc test, a meropenem disc with EDTA gives a zone at least 7 mm larger than meropenem alone, and ceftazidime-avibactam is inactive. Which resistance mechanism is most likely?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. A zinc-dependent metallo-beta-lactamase such as NDM

Metallo-beta-lactamases such as NDM need zinc at the active site, so the chelator EDTA restores carbapenem activity in a combined disc test. They are not inhibited by clavulanate, tazobactam or avibactam. NDM is common in carbapenem-resistant Enterobacterales in India, and combination such as aztreonam with ceftazidime-avibactam is used in treatment.

Option by option

  • A. A serine carbapenemase such as KPC

    KPC is a serine enzyme that avibactam inhibits and EDTA does not affect.

  • B. A zinc-dependent metallo-beta-lactamase such as NDM (correct)

    Zinc dependence explains the EDTA effect and the lack of avibactam activity.

  • C. An extended-spectrum beta-lactamase only

    ESBLs do not hydrolyse carbapenems, so meropenem would remain active.

  • D. Altered penicillin-binding protein PBP2a

    PBP2a is the mecA product of MRSA and does not occur in Klebsiella.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, A zinc-dependent metallo-beta-lactamase such as NDM. Metallo-beta-lactamases such as NDM need zinc at the active site, so the chelator EDTA restores carbapenem activity in a combined disc test.

Why is "A serine carbapenemase such as KPC" not the answer?

KPC is a serine enzyme that avibactam inhibits and EDTA does not affect.

Why is "An extended-spectrum beta-lactamase only" not the answer?

ESBLs do not hydrolyse carbapenems, so meropenem would remain active.

Why is "Altered penicillin-binding protein PBP2a" not the answer?

PBP2a is the mecA product of MRSA and does not occur in Klebsiella.

What concept does this question test?

It tests Antimicrobial resistance: metallo-beta-lactamase in Microbiology. They are not inhibited by clavulanate, tazobactam or avibactam. NDM is common in carbapenem-resistant Enterobacterales in India, and combination such as aztreonam with ceftazidime-avibactam is used in treatment.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Antimicrobial resistance: metallo-beta-lactamase?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.